Natural or innate immunity plays an important role in combating infectious disease and killing of tumor cells. The long term goal of this project is to study the broad class of animal lectin-mediated recognition processes and to elucidate the mechanisms through which host defense is mediated by this 'primitive' or perhaps primary immune function. The specific aim of this project is to examine the structure and regulation of the genes encoding two vertebrate lectins, the mannose-binding proteins, and use this information to obtain embryonal stem cell lines with targeted gene disruption for development of chimeric mice deficient in synthesis of these proteins. The chimeric model would offer a better means to understand their in vivo physiological roles. Mannan/mannose binding proteins (MBP) are calcium-dependent lectins synthesized in liver and secreted into serum. The evidence that human MBP plays an important role in vivo has come from studies on certain children with low circulating levels of MBP, who suffer frequent bacterial and yeast infections. An autosomal dominant mutation in the gene encoding MBP that could disrupt its structure and abrogate function was demonstrated recently in such immune-deficient children from three families. Two homologous, yet distinct forms of MBP, are known in rodents, MBP-A and MBP-C. In humans, only one form, the equivalent of murine MBP-A is known. The physiological role of murine MBP-C is unknown. In the proposed project, 1) previously isolated genes encoding mouse MBP-A and C will be characterized by mapping, subcloning and sequencing, and determining their chromosomal location; 2) cis-acting regulatory elements controlling the upregulation of MBP-A mRNA during the acute phase response will be defined; 3) a comparative in vitro study on the opsonization of pathogens with mouse MBPs and their interaction with phagocytes will be made by using recombinant proteins obtained by expression of the cDNAs in Chinese hamster ovary cells; and finally, 4) embryonal stem cell lines with targeted disruptions in MBP-A or MBP-C gene, by homologous recombination, will be isolated and injected into mouse blastocysts to develop chimeric mice deficient in MBP-A/C protein. These studies should i) help in understanding the structure and evolutionary relationship of two homologous vertebrate lectin genes and aid in the development of a murine model of human immunodeficiency disorder, ii) provide information on the mechanisms regulating the MBP-A gene in response to acute phase stimuli, and iii) be useful in defining the fundamental biological roles of the two murine mannose-binding proteins.